Nitrogen oxides, or NOx, is the generic term for a group of highly reactive gases, all of which contain nitrogen and oxygen in varying amounts. Many of the nitrogen oxides are colorless and odorless. However, one common pollutant, nitrogen dioxide (NO2) along with particles in the air can often be seen as a reddish-brown layer over many urban areas. Nitrogen oxides form when fuel is burned at high temperatures, as in a combustion process. The primary manmade sources of NOx are motor vehicles, electric utilities, and other industrial, commercial, and residential sources that burn fuels. NOx can also be formed naturally.

Nitrogen oxides are indirect greenhouse gases. They have been the target of environmental policies for their role in forming ozone (O3), as well for their direct acidification effects. Fuel combustion activities are the most significant anthropogenic source of NOx. Within fuel combustion, the most important sources are the energy industries and mobile sources.